We previously demonstrated specific in vitro killing of murine tumor cells by the combined action of lymphoid cells from non-sensitized syngeneic mice and heat-decomplemented sera from mice bearing the specific tumor (Moloney virus- or 3-methylcholonthrene-induced sarcomas or mammary tumor virus-induced adenocarcinomas). Cytotoxicity is not mediated by either the serum or lymphoid cells alone. Cytotoxicity by serum plus lymphoid cell can be demonstrated in a microcytotoxicity test by (1) pretreating plated tumor cells with serum prior to addition of the lymphoid cells or (a) by arming lymphoid cells with serum and adding the armed, washed cells to the plated tumor cells. We are now studying (a) the nature of the serum component(s) by use of affinity chromatography; (b) the identity of the effector cell by means of affinity columns and specific anti-sera; (c) the effects of arming serum or of armed cells on tumor growth in vivo.